Mulching machines for mulching wood and machines for shredding objects such as boat hulls and tree trunks have a rotatable drum with a plurality of cutting tools mounted on the drum. The cutting tools are symmetric about a longitudinal axis and have a tapered forward cutting end for shredding the material and a cylindrical shank that is rotatably mounted in the cylindrical bore of a tool holder on the machine. The tool must be mounted to rotate as the machine operates so that the tapered forward cutting end of the tool wears evenly around its circumference. If the tool fails to rotate, the tool will wear unevenly and its useful life will be significantly shortened. When a number of tools on the machine become worn, the machine must be taken out of service and the damaged tools removed and replaced.
The machines that mulch up bulky material have drums that rotate rapidly such that the surface speed of the drum is as high as six thousand feet per minute. If the rotating teeth on the drum encounter a particularly hard substance, such as a rock or large piece of metal, the impact may suddenly stop the rotation of the drum. When the drum jams, the tools mounted on the periphery of the drum are decelerated in a fraction of a second and as a result, a powerful force is applied to pull the tool out of the tool holder.
In the past, it has been the practice to retain such tools in the tool holders of the machines by providing a deep groove around the circumference of the shank of the tool and fitting spring pins through the tool holder so as to engage the groove in the shank and thereby retain the shank into the tool holder. It has been found, however, that over a period of time the forward end of the tool holder becomes worn away by a radial flange on the tool as the tool rotates. The results of the wearing of the forward surface of the tool holder is that the shank of replacement tools fits deeper into the bore of the tool holder and the annular groove on the shank is no longer aligned with the spring pin that retains the tool in the tool holder. The consequence is that continued rotation of the tool causes the spring pin to be worn away. As a result, the instance of tool failure, where the tool is lost out of its retainer, occurs more frequently as the tool holders of the machine become worn.
It would be desirable, therefore, to provide an improved tool holder that would retain the tool against a strong force pulling the tool out of the holder. It would also be desirable to provide a tool holder that would not become compromised as the tool holders of the machine become worn.